An optical nor gate is formed from two pair of optical waveguide devices on a substrate, with each pair of the optical waveguide devices consisting of an electroabsorption modulator electrically connected in series with a waveguide photodetector. The optical nor gate utilizes two digital optical inputs and a continuous light input to provide a nor function digital optical output. The optical nor gate can be formed from iii-V compound semiconductor layers which are epitaxially deposited on a iii-V compound semiconductor substrate, and operates at a wavelength in the range of 0.8-2.0 μm.
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1. A photonic integrated circuit which generates a nor function digital optical output from a pair of digital optical inputs, comprising:
a first pair of optical waveguide devices formed on a substrate, with the first pair of optical waveguide devices comprising a first electroabsorption modulator which is electrically connected in series with a first photodetector, with the first electroabsorption modulator receiving an input of continuous light, and with the first photodetector receiving a first digital optical input of the pair of digital optical inputs to generate therefrom a photocurrent signal which changes a reverse-bias voltage on the first electroabsorption modulator to generate a digitally-modulated output from the input of continuous light; and
a second pair of optical waveguide devices formed on the substrate, with the second pair of optical waveguide devices comprising a second electroabsorption modulator which is electrically connected in series with a second photodetector, with the second electroabsorption modulator receiving the digitally-modulated output from the first electroabsorption modulator, and with the second photodetector receiving a second digital optical input of the pair of digital optical inputs to generate therefrom another photocurrent signal which changes the reverse bias voltage on the second electroabsorption modulator to convert the digitally-modulated output into the nor function digital optical output.
12. An optical nor gate which receives a first digital optical input and a second digital optical input and generates therefrom a nor function digital optical output, comprising:
a iii-V compound semiconductor substrate having a plurality of iii-V compound semiconductor layers epitaxially grown thereon;
a laser formed from the plurality of iii-V compound semiconductor layers to provide a source of continuous light;
a first electroabsorption modulator formed from the plurality of iii-V compound semiconductor layers, with the first electroabsorption modulator receiving the continuous light from the laser;
a first waveguide photodetector formed from the plurality of iii-V compound semiconductor layers to receive the first digital optical input and to generate therefrom a first photocurrent signal which changes an absorption of light in the first electroabsorption modulator, thereby modulating the continuous light transmitted through the first electroabsorption modulator to provide a digitally-modulated output from the first electroabsorption modulator;
a second electroabsorption modulator formed from the plurality of iii-V compound semiconductor layers, with the second electroabsorption modulator receiving the digitally-modulated output from the first electroabsorption modulator; and
a second waveguide photodetector formed from the plurality of iii-V compound semiconductor layers to receive the second digital optical input and to generate therefrom a second photocurrent signal which changes the absorption of light in the second electroabsorption modulator, thereby converting the digitally-modulated output being transmitted through the second electroabsorption modulator into the nor function digital optical output.
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This invention was made with Government support under Contract No. DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
The present application is related to U.S. patent application Ser. No. 12/182,683, entitled “Optical NAND Gate,” of common assignee filed on Jul. 30, 2008, the contents of which are incorporated herein by reference in their entirety.
The present invention relates in general to digital optical logic gates, and in particular to a optical NOR gate which utilizes electroabsorption modulators and waveguide photodetectors to generate a NOR function digital optical output from a pair of digital optical inputs.
Optical logic gates have been the subject of research for several decades due to the possibility of achieving higher operating speeds than logic based on electronics. The advantages of digital signal processing in the optical domain include higher signal bandwidth, lower signal cross-talk, and greater protection against electronic eavesdropping. All-optical signal processing also eliminates the need to convert signals from the optical domain into the electronic domain for processing and then to re-convert the processed signals from the electronic domain back into the optical domain. All-optical signal processing is advantageous to reduce the cost, electrical power requirement, size and weight compared to optical-to-electronic converters, electronic signal processing circuitry, and electronic-to-optical converters which are currently being used.
The present invention addresses the need for optical logic gates by providing an optical NOR gate which can be formed as a photonic integrated circuit (PIC) using two electroabsorption modulator (EAM) photodiode (PD) pairs, with each EAM/PD pair being electrically connected in series. This configuration according to the present invention provides advantages in terms of optical isolation of input and output signals, an ability to be monolithically integrated and an ability to operate using direct-current electrical power sources with a relatively low power consumption and a relatively compact size. The present invention is also advantageous in providing for optical signal gain and regeneration thereby permitting a fan out capability which can allow multiple optical NOR gates to be interconnected together or to be interconnected to the optical NAND gates described in U.S. patent application Ser. No. 12/182,683 to provide a higher level of logic functionality as needed for optical signal processing or optical computing applications.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to a photonic integrated circuit (PIC) which generates a NOR function digital optical output (also referred to herein as a NOR function output or a NOR output) from a pair of digital optical inputs. The PIC comprises a first pair of optical waveguide devices formed on a substrate, with the first pair of optical waveguide devices comprising a first electroabsorption modulator which is electrically connected in series with a first photodetector, and a second pair of optical waveguide devices formed on the substrate, with the second pair of optical waveguide devices comprising a second electroabsorption modulator which is electrically connected in series with a second photodetector. The first electroabsorption modulator receives an input of continuous light; and the first photodetector receives a first digital optical input of the pair of digital optical inputs and generates therefrom a photocurrent signal which changes a reverse-bias voltage on the first electroabsorption modulator to generate a digitally-modulated output from the input of continuous light. The second electroabsorption modulator receives the digitally-modulated output from the first electroabsorption modulator; and the second photodetector receives a second digital optical input of the pair of digital optical inputs and uses the second digital optical input to generate another photocurrent signal which changes the reverse bias voltage on the second electroabsorption modulator to convert the digitally-modulated output into the NOR function digital optical output.
The input of continuous light can be provided by a laser which can be located on the substrate. The laser can comprise a distributed-Bragg reflector (DBR) laser. The laser can operate in a wavelength range of 0.8-2.0 microns, with the pair of digital optical inputs also being in this same wavelength range.
A plurality of optical waveguides can be provided on the substrate to guide the input of continuous light to the first electroabsorption modulator, to guide the first digital optical input to the first photodetector, to guide the digitally-modulated output from the first electroabsorption modulator to the second electroabsorption modulator, to guide the second digital optical input to the second photodetector, and to guide the NOR function digital optical output from the second electroabsorption modulator.
The substrate can comprise a III-V compound semiconductor substrate. Each electroabsorption modulator and each photodetector can comprise a plurality of III-V compound semiconductor layers which are epitaxially grown on the III-V compound semiconductor substrate.
In certain embodiments of the present invention, the III-V compound semiconductor substrate can comprise indium phosphide (InP); and the plurality of III-V compound semiconductor layers can be selected from the group consisting of indium gallium arsenide phosphide (InGaAsP) layers, indium gallium arsenide (InGaAs) layers, indium aluminum gallium arsenide (InAlGaAs) layers, and combinations thereof. In other embodiments of the present invention, the III-V compound semiconductor substrate can comprise gallium arsenide (GaAs); and the plurality of III-V compound semiconductor layers can be selected from the group consisting of GaAs layers, aluminum gallium arsenide (AlGaAs) layers, InGaAsP layers, InGaAs layers, and combinations thereof.
One or more semiconductor optical amplifiers (SOAs) can be optionally formed on the substrate to amplify one or more optical signals selected from the group consisting of the input of continuous light, the first digital optical input, the second digital optical input, the digitally-modulated output from the first electroabsorption modulator, and the NOR function digital optical output.
The present invention also relates to an optical NOR gate which receives a first digital optical input and a second digital optical input and generates therefrom a NOR function digital optical output. The optical NOR gate comprises a III-V compound semiconductor substrate having a plurality of III-V compound semiconductor layers epitaxially grown thereon; a laser formed from the plurality of III-V compound semiconductor layers to provide a source of continuous light; a first electroabsorption modulator formed from the plurality of III-V compound semiconductor layers, with the first electroabsorption modulator receiving the continuous light from the laser; a first waveguide photodetector formed from the plurality of III-V compound semiconductor layers to receive the first digital optical input and to generate therefrom a first photocurrent signal which changes an absorption of light in the first electroabsorption modulator, thereby modulating the continuous light transmitted through the first electroabsorption modulator to provide a digitally-modulated output from the first electroabsorption modulator; a second electroabsorption modulator formed from the plurality of III-V compound semiconductor layers, with the second electroabsorption modulator receiving the digitally-modulated output from the first electroabsorption modulator; and a second waveguide photodetector formed from the plurality of III-V compound semiconductor layers to receive the second digital optical input and to generate therefrom a second photocurrent signal which changes the absorption of light in the second electroabsorption modulator, thereby converting the digitally-modulated output being transmitted through the second electroabsorption modulator into the NOR function digital optical output.
The first digital optical input and the second digital optical input can have a wavelength in the range of 0.8-2.0 microns; and the continuous light from the laser can also have a wavelength in the range of 0.8-2.0 microns. The wavelengths of the first and second digital optical inputs can be the same or different; and the wavelength of the continuous light from the laser can also be the same or different from one or both of the first and second digital optical inputs.
In some embodiments of the present invention, the III-V compound semiconductor substrate can comprise indium phosphide (InP); and the plurality of III-V compound semiconductor layers can be selected from the group consisting of indium gallium arsenide phosphide (InGaAsP) layers, indium gallium arsenide (InGaAs) layers, indium aluminum gallium arsenide (InAlGaAs) layers, and combinations thereof. In other embodiments of the present invention, the III-V compound semiconductor substrate can comprise gallium arsenide (GaAs); and the plurality of III-V compound semiconductor layers can be selected from the group consisting of GaAs layers, aluminum gallium arsenide (AlGaAs) layers, InGaAsP layers, InGaAs layers, and combinations thereof. A plurality of passive optical waveguides can be provided on the substrate and formed from the plurality of III-V compound semiconductor layers. These passive optical waveguides can be used to guide continuous light from the laser to the first electroabsorption modulator, to guide the first digital optical input to the first waveguide photodetector, to guide the second digital optical input to the second waveguide photodetector, to guide the digitally-modulated output from the first electroabsorption modulator to the second electroabsorption modulator, and to guide the NOR function digital optical output from the second electroabsorption modulator.
The first electroabsorption modulator and the first waveguide photodetector are electrically connected in series; and the second electroabsorption modulator and the second waveguide photodetector are also electrically connected in series. The laser can comprise a distributed Bragg reflector (DBR) laser.
One or more semiconductor optical amplifiers can be optionally formed on the III-V compound semiconductor substrate from the plurality of III-V compound semiconductor layers to amplify one or more optical signals. These optical signals can be selected from the group consisting of the continuous light from the laser, the first digital optical input, the second digital optical input, the digitally-modulated output from the first electroabsorption modulator, and the NOR function digital optical output.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings:
Referring to
The optical NOR gate 10 comprises a substrate 12 on which are formed a first electroabsorption modulator (EAM) 14 and a first waveguide photodetector (PD) 16 which are electrically connected together in series with a cathode side of the PD 16 being connected to an anode side of the EAM 16 as shown in
In
The laser 22 can be a distributed Bragg reflector (DBR) laser 22 which is located on the substrate 12 as shown in
The first waveguide photodetector 16 receives a first digital optical input 110, which is denoted as the “A” input. The “A” input 110 can be provided, for example, from an optical fiber and can comprise a stream of digital optical data. The “A” input 110 is transmitted through another passive optical waveguide 24 (also referred to herein as an optical waveguide 24) on the substrate 12 and is directed into the first waveguide photodetector 16 where the “A” input 110 is absorbed to generate a photocurrent signal which can be used to modulate an absorption of the continuous light input 100 being transmitted through the first electroabsorption modulator 14, thereby producing a digitally-modulated output 120 from the first electroabsorption modulator 14.
The second waveguide photodetector 20 in
Since the “A” and “B” inputs 110 and 130, respectively, are digital signals, then the photocurrent signal generated by each photodetector 16 and 20 will also generally be digital signals depending upon a data rate for operation of the device 10 which can be up to 100 gigaHertz (GHz) or more. Any of the “A” and “B” inputs 110, 130 which are not absorbed in the photodetectors 16, 20 to generate the photocurrent signals are coupled into L-shaped passive waveguides 28 which serve as optical traps.
The first electroabsorption modulator 14 and the first waveguide photodetector 16 are electrically connected in series by wiring 30 formed on the substrate 12. The wiring 30, which can be in the form of a radio-frequency (rf) transmission line, connects an upper electrode 32 of the first electroabsorption modulator 14 to an lower electrode 34′ of the first waveguide photodetector 16 to connect these two elements 14 and 16 in series. This same wiring 30 is connected through a resistor 36 (e.g. a 20-50 Ohm resistor) to an electrical ground connection which is in common with a first bias voltage V1 which is used to reverse-bias the first electroabsorption modulator 14 and which is also in common with a second bias voltage V2 which is used to reverse-bias the first waveguide photodetector 16. Additional wiring 30 is used to connect a lower electrode 32′ of the first electroabsorption modulator 14 to the bias voltage V1, and to connect an upper electrode 34 of the first waveguide photodetector 16 to the bias voltage V2. The first bias voltage V1 can be, for example, −1 Volt; and the second bias voltage V2 can be, for example, −5 Volts. The bias voltages V1 and V2 can be supplied by direct-current (dc) power supplies which can be located off of the substrate 12 as shown in
In the example of
By design, the photocurrent signal generated by the first waveguide photodetector 16 in response to incident light (i.e. the “A” input 110) is relatively independent of an electric field produced therein by the applied reverse-bias voltage V2 and depends only upon the intensity of the incident light. This is also the case for the second waveguide photodetector 20.
On the other hand, the absorption within the first electroabsorption modulator 14 depends upon the electric field produced therein by the applied reverse-bias voltage V1 and any portion of the bias voltage V2 which is dropped across the modulator 14 as a result of the photocurrent generated by the first waveguide photodetector 16. As the amount of the reverse-bias voltage dropped across the electroabsorption modulator 14 increases in response to the photocurrent from the photodetector 16, the absorption of light in the first electroabsorption modulator 14 will increase either due to a Franz-Keldysh effect, or due to a quantum-confined Stark effect. This is also the case for the second electroabsorption modulator 18 with an absorption that is dependent on the photocurrent generated by the second waveguide photodetector 20 as will be described hereinafter. The circuit arrangement of
In the absence of any light input into the first waveguide photodetector 16 due to the “A” input 110 being in a logical “0” state, no photocurrent signal will be generated from the photodetector 16 so that a node “C” where the photodetector 16, modulator 14 and resistor 36 are all connected together (see
Conversely, when the “A” input 110 is in a logical “1” state corresponding to a relatively high light level, the absorption of the “A” input 110 in the first waveguide photodetector 14 will generate a relatively large photocurrent. This photocurrent will flow through the resistor 36 thereby producing a relatively large change in the electrical potential at node “C” due to the reverse-bias voltage V2 (e.g. −5 Volts) being much larger than V1 (e.g. −1 Volt). The change in the electrical potential at node “C” will add to V1 to increase an overall amount of reverse-bias voltage which is dropped across the first electroabsorption modulator 14 from about one Volt, for example, to several Volts. This increase in the overall amount of the reverse-bias voltage dropped across the first electroabsorption modulator 14 will greatly increase the absorption of the continuous light 100 therein so that very little, if any, of the continuous light 100 will be transmitted through the modulator 14, thereby providing a very low light level for the digitally-modulated output 120 corresponding to the logical “0” state.
The digitally-modulated output 120 is transmitted through the optical waveguide 24 to the second electroabsorption modulator 18 which is connected in series to the second waveguide photodetector 20 and to another resistor 36′ (e.g. a 20-50 Ohm resistor) at a node “D” as shown in
The “B” input 130 is coupled through one of the passive optical waveguides 24 to the second waveguide photodetector 20 and is used to control the operation of the second electroabsorption modulator 18 in the same way that the first electroabsorption modulator 14 is controlled by the photocurrent generated by the first waveguide photodetector 16. The second waveguide photodetector 20 includes an upper electrode 40 and a lower electrode 40′ as shown in
When the “B” input 130 is in the logical “0” state, no photocurrent signal will be generated from the second waveguide photodetector 20 so that node “D” will be at about ground electrical potential and the entire reverse-bias voltage V1 dropped across the second electroabsorption modulator 18. Since V1 is relatively small as previously discussed, there will be a relatively small absorption of the digitally-modulated output 120 within the second electroabsorption modulator 18 so that the NOR output 140 will be substantially equal to the digitally-modulated output 120 (i.e. when the digitally-modulated output 120 is in the “0” state, the NOR output 140 will be in the “0” state; and when the digitally-modulated output 120 is in the “1” state, the NOR output 140 will be in the “1” state).
When the “B” input 130 is in the logical “1” state, a relatively large photocurrent will be generated in the second waveguide photodetector 20 and will flow through the resistor 36′. This will increase the electrical potential at the node “D” due to the higher reverse-bias voltage V2 (e.g. −5 Volts); and will also add to V1 to increase the overall amount of reverse-bias voltage being dropped across the second electroabsorption modulator 14, thereby greatly-increasing the absorption of light therein. Thus, the NOR output 140 will be switched to a relatively low level corresponding to the logical “0” state regardless of the logic state of the digitally-modulated output 120 which is input into the second electroabsorption modulator 18. A logical truth table for the optical NOR gate 10 is shown in
Fabrication of the optical NOR gate 10 of
Those skilled in the art will understand that the optical NOR gate 10 of the present invention can also be fabricated using other types of III-V compound semiconductor fabrication methods which are well-known in the art. These other types of III-V compound semiconductor fabrication methods include butt-joint regrowth, selective area growth, and the use of offset quantum wells and are detailed in the following articles which are incorporated herein by reference: E. Skogen et al., “Monolithically Integrated Active Components: A Quantum-Well Intermixing Approach,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 11, pp. 343-355, March/April 2005; J. W. Raring et al., “40-Gb/s Widely Tunable Transceivers,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 13, pp. 3-14, January/February 2007.
In
An ion implant mask (e.g. comprising silicon nitride about 0.5 μm thick) can then be provided over the substrate 12 and III-V compound semiconductor layers with openings at locations wherein phosphorous ions are to be implanted into the InP implant buffer layer 62 for use in selectively disordering the MQW region 50. The locations where the waveguide photodetectors 16 and 20 and a gain region of the laser 22 are to be formed will generally not have a disordered MQW region 50 since the MQW region 50 is epitaxially grown to optimize the performance of the photodetectors 16 and 20 and the gain region of the laser 22. The phosphorous ions can be implanted into the InP implant buffer layer 62 at an ion energy of about 100 keV and a dose of about 5×1014 cm−2 with the substrate 12 being at a temperature of about 200° C. The implanted phosphorous ions produce point defects in the InP implant buffer layer 62.
A rapid thermal annealing step can then be used to drive the point defects down into the MQW region 50 to intermix the QW layers 52 and the barrier layers 54 at the interfaces therebetween. This intermixing produces a blue-shift the energy bandgap wavelength in the MQW region 50. The rapid thermal annealing step can be performed at a temperature in the range of 630-700° C. and can be timed for a time interval from about one-half minute up to a few tens of minutes to provide a predetermined energy bandgap wavelength for the MQW region 50 which will depend upon the exact elements of the optical NOR gate 10 being formed. To form the electroabsorption modulators 14 and 18, a first rapid thermal annealing step can be used to provide a few tens of nanometer blue-shift in the energy bandgap wavelength of the MQW region 50 (e.g. to μg˜1.50 μm) to reduce an absorption loss therein in the absence of any reverse-bias voltage. This same blue-shift is provided for the MQW region 50 for the passive waveguides 24 and for distributed Bragg reflector (DBR) mirror regions which are used to form an optical cavity for the DBR laser 22 and for the gain region and an optional phase control region located within the optical cavity of the DBR laser 22. An additional blue-shift will be provided in a subsequent thermal annealing step for the passive waveguides 24 and the DBR mirror regions to further increase the blue-shift therein (e.g. to λg˜1.43 μm) and thereby further reduce the absorption for these elements of the optical NOR gate 10. The blue-shift in the energy bandgap wavelength of the MQW region 50 can be determined using a laser-excited photoluminescence spectroscopy measurement at room-temperature.
After the first rapid thermal annealing step, the InP implant buffer layer 62 can be removed above the electroabsorption modulators 14 and 18 while leaving the layer 62 in place over the passive waveguides 24 and DBR mirror regions. This can be done using a wet etching step to etch away the layer 62, with the wet etching being terminated upon reaching the InGaAsP etch stop layer 60. Removal of the InP implant buffer layer 62 above the electroabsorption modulators 14 and 18 prevents any further blue-shift in the MQW region 50 at these locations since it removes the source of point defects necessary for quantum-well intermixing.
A second rapid thermal annealing step can then be performed at about the same temperature for up to a few minutes (e.g. 2-3 minutes) to provide further intermixing of the QW and barrier layers 52 and 54 to produce an additional few tens of nanometers blue-shift the energy bandgap of the MQW region 50 in the remaining regions where the InP implant buffer layer 62 is still present. This additional blue-shift in the energy bandgap of the MQW region 50 further reduces the absorption loss in the passive waveguides 24 and the DBR mirror regions of the laser 22 in the optical NOR gate 10.
After the second rapid thermal annealing step is performed, the remaining InP implant buffer layer 62 and the InGaAsP etch stop layer 60 can be completely removed from the substrate 12 by wet etching. This is schematically illustrated in the cross-section view of
A blanket MOCVD regrowth can then be performed to epitaxially grow an upper cladding layer 64 of p-type-doped InP which can be, for example, 2.35 μm thick followed by a cap layer 66 of p-type-doped InGaAs about 0.2 μm thick. This is shown in
In other embodiments of the present invention, an offset quantum-well region can be epitaxially grown above the upper waveguide layer 56. This can be useful to form the photodetectors 16 and 20 as uni-traveling carrier photodetectors, and can also be useful to form semiconductor optical amplifiers (SOAs). The use of an offset quantum-well region provides a lower confinement factor than the quantum-well region 50 and thus can increase the saturation power level for the photodetectors 16 and 20 and any SOAs and also allow operation at higher frequencies. Further details of the fabrication of photodetectors and SOAs using offset quantum-well region can be found in an article by J. W. Raring et al., “Design and Demonstration of Novel QW Intermixing Scheme for the Integration of UTC-Type Photodiodes with QW-Based Components,” IEEE Journal of Quantum Electronics, vol. 42, pp. 171-181, February 2006, which is incorporated herein by reference.
An etch mask (not shown) can be provided over the substrate 12 and photolithographically patterned for use in etching down through the InGaAs cap layer 66 and the InP upper cladding layer 64 as shown in
In
In
In
Adjacent elements of the optical NOR gate 10, which are not optically connected, can be electrically isolated by etching down partway into the semi-insulating InP substrate 12 as shown in
In this third example of the present invention, a pair of semiconductor optical amplifiers (SOAs) 76 are provided on the substrate 12 to amplify the “A” input 110 and the “B” input 130 prior to detection by the first and second waveguide photodetectors 16 and 20, respectively. This is useful to provide an increased optical power (e.g. up to a few tens of milliWatts) for the amplified “A” and “B” inputs, which will provide larger photocurrent signals from the photodetectors 16 and 20, and thereby provide a larger on/off contrast for the digitally-modulated output 120 from the first electroabsorption modulator 14 and for the NOR output 140 from the second electroabsorption modulator 18. The provision of the SOAs 76 in the optical NOR gate 10 of
Each SOA 76 can be formed using a gain region similar to the gain region in the laser 22 with an upper electrode 78 and a lower electrode 78′. The gain region for each SOA 76 can have a length of, for example, 100-500 μm, and a width of, for example, 1-10 μm. Alternatively, each SOA 76 can be formed as flared SOA 76 with a width that increases along the length of the SOA 76, or with an offset multi-quantum-well (MQW) gain region. The use of a flared SOA 76 or an SOA 76 having an offset MQW gain region is useful to provide a higher saturation power level for the SOA 76. The SOAs 76 in the example of
In the example of
In the absence of any light input into the first waveguide photodetector 16 due to the “A” input 110 being in a logical “0” state, no photocurrent signal will be generated from the photodetector 16 so that node “C” where the photodetector 16, modulator 14 and resistor 36 are all connected together will be at about ground electrical potential and substantially the entire reverse-bias voltage V1 (e.g. −1 Volt) will be dropped across the first electroabsorption modulator 14. Since V1 is relatively small, the absorption of the continuous light input 100 in the first electroabsorption modulator 14 will be relatively small with substantially all of the continuous light input 100 being transmitted through the modulator 14 so that the digitally-modulated output 120 from the first electroabsorption modulator 14 will be relatively large corresponding to the logical “1” state.
Conversely, when the “A” input 110 is in a logical “1” state, the “A” input 110 will be amplified by the SOA 76 prior to being detected by the first waveguide photodetector 16. This will produce a relatively large photocurrent in the first waveguide photodetector 16 when the amplified “A” input 110 is detected, with this photocurrent flowing through the resistor 36 to ground. Since the reverse-bias voltage V2 (e.g. −5 Volts) is generally larger than V1, the photocurrent signal generated by the photodetector 16 will produce a large change in the electrical potential at node “C” which will add to V1, thereby substantially increasing the overall reverse-bias voltage which is dropped across the first electroabsorption modulator 14. This will greatly increase the absorption of the continuous light 100 within the modulator 14 so that the continuous light 100 will be substantially all absorbed therein to provide a very low light level for the digitally-modulated output 120 corresponding to the logical “0” state.
When the digitally-modulated output 120 reaches the second electroabsorption modulator 18, the absorption of the digitally-modulated output 120 therein will again depend upon the photocurrent generated by the second waveguide photodetector 20. With the modulator 18 and photodetector 20 connected in series as shown in
When the “B” input 130 is in the logical “1” state, the input 130 will be amplified by the SOA 76 so that a relatively large photocurrent will be generated in the second waveguide photodetector 20. This will increase the reverse-bias voltage across the second electroabsorption modulator 14 thereby greatly increasing the absorption of the digitally-modulated output 120 therein to provide a logical “0” state for the NOR output 140 regardless of the logic state of the digitally-modulated output 120 which is input into the second electroabsorption modulator 18.
Those skilled in the art will understand that the optical NOR gate 10 of the present invention is a base logic gate that can be used to implement any other logic function. The optical NOR gate 10 of the present invention can be used alone to digitally process optical information, or as a building block to form an optical logic circuit, an optical signal processor, an optical computer, etc. A plurality of optical NOR gates 10 can be formed on a common substrate 12 and optically connected with passive waveguides 24 to other types of optical logic gates such as the optical NAND gates described in U.S. patent application Ser. No. 12/182,683, which is incorporated herein by reference, in a way analogous to the interconnection of a plurality of transistors to form an integrated circuit. Thus, for example, to form an optical signal processor or optical computer, the NOR outputs 140 from a plurality of optical NOR gates 10 would be used to provide the “A” and “B” inputs for other devices of the same or different type to provide a higher level of complexity as needed for the optical signal processor or optical computer.
The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.
Tauke-Pedretti, Anna, Skogen, Erik J.
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